Japanese Patent Application Laid-Open No. 2012-230657 discloses a display panel in which a top surface polarizing plate is disposed on an operation surface (outermost surface). In this display panel, a touch panel is disposed on a surface opposite to the operation surface of the top surface polarizing plate. A liquid crystal panel formed by a pair of glasses sandwiching a liquid crystal is disposed on the side opposite to the top surface polarizing plate of the touch panel.
Japanese Patent Application Laid-Open No. 2015-095696 discloses a display device which detects a pressing operation (i.e., a pressing force on the touch panel, typically a force which is greater than a predetermined value) using a uniaxially stretched piezoelectric sheet.
However, a piezoelectric sheet produces birefringence when uniaxially stretched. Therefore, when, for example, a display image is viewed with polarizing sunglasses on, a phase difference of the piezoelectric sheet changes wavelength characteristics of the light intensity, and changes a tone of images.
For example, by making a polarization direction of the top polarizing plate of the display panel and the uniaxial stretching direction of the piezoelectric sheet parallel or orthogonal, it is possible to eliminate the influence of the phase difference of the piezoelectric sheet on the optical characteristics.
However, due to manufacturing variations of the piezoelectric sensor, manufacturing variations of the display panel, or assembly variations of the piezoelectric sensor and the display panel, a parallel or orthogonal state of the polarization direction of the top polarizing plate of the display panel and the stretching direction of the piezoelectric sheet may not be realized. Further, the stretching direction of the piezoelectric sheet which is optimal for detecting a pressing operation, and the polarization direction of the top polarizing plate of the display panel are not necessarily in the parallel or orthogonal state. When the polarization direction of the top polarizing plate of the display panel and the stretching direction of the piezoelectric sheet are not in the parallel or orthogonal state, a tone of images changes when a displayed image is viewed with the polarization sunglasses on.